1. Field of the Invention
This invention relates generally to the transmission of digital data over optical fibers, and more particularly, to transmission based on quadrature amplitude modulation (QAM) and frequency division multiplexing.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the areas of networking including the Internet, telecommunications, and application areas which rely on networking or telecommunications, there is an increasing demand for capacity for the transmission of digital data. For example, the transmission of digital data over a network""s trunk lines (such as the trunk lines for telephone companies or for the Internet), the transmission of images or video over the Internet, the distribution of software, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically requires the high speed transmission of large amounts of digital data. Typical protocols which are intended to support such transmissions include the OC, STM, and STS protocols. As applications such as the ones mentioned above become more prevalent, the use of these and similar protocols and the corresponding demand for transmission capacity will only increase.
Optical fiber is a transmission medium which is well-suited for the high speed transmission of digital data. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable, and protocols such as the OC protocol have been developed for the transmission of digital data over optical fibers. However, increasing the data throughput of an optical fiber simply by increasing the clock speed of these protocols, such as moving from 155 million bits per second (Mbps) OC-3 to 625 Mbps OC-12, is not straightforward.
For example, existing optical fiber communication systems typically use simple modulation schemes which result in low bandwidth efficiencies of approximately 1 bit per sec per Herz (bps/Hz). As an example, the OC protocol is based on on-off keying (OOK), which is a bandwidth inefficient modulation scheme, and the transmission of OC signals across optical fiber results in a bandwidth efficiency of approximately 1 bps/Hz. The useable bandwidth of current optical fibers is limited in part by dispersion and non-linearities which increase with bandwidth. The low bandwidth efficiency means that, for a given digital data rate, the transmitted signal will occupy a larger bandwidth. This results in larger dispersion and non-linear effects, which limit the useful transmission range of the system.
In addition, even if the optical fiber supports the higher data rates, the corresponding electronics and electro-optics might not be able to. For example, moving from OC-3 to OC-12 quadruples the bit rate but also requires the associated electronics to operate approximately four times faster. Electronics at these speeds simply may not be available or, if available, may have significant other drawbacks, such as larger power consumption, unwieldy size, high cost, or unacceptable fragility.
In theory, the bandwidth efficiency problem could be addressed partly by the use of more bandwidth-efficient modulation schemes, such as quadrature amplitude modulation (QAM). These modulation schemes have been used previously in radio-wave and coaxial systems. However, optical fiber systems are based on an entirely different technology base and many of the technologies, techniques, and design tradeoffs which were developed in order to implement more advanced modulation schemes in radio-wave and coaxial systems would have only minimal application to optical fiber systems. In addition, optical fiber systems present their own difficulties, such as fiber dispersion and non-linearities causing unwanted interference. Even if bandwidth-efficient modulation schemes could be easily applied to optical fiber systems, their use does not fully address the high-speed electronics problem described above. For example, if OC data streams were QAM-modulated rather than OOK-modulated, a move from OC-3 to OC-12 would still require a four-fold increase in the speed of the corresponding electronics.
As a result, the application of sophisticated modulation schemes to optical fiber systems has been limited. For example, QAM has recently been applied to an optical fiber system for the transmission of compressed video for the cable TV industry. However, these communications systems run at low speeds with an aggregate data rate of less 1 billion bits per second (Gbps). Hence, they are not suited for high speed optical network operation.
Wavelength division multiplexing (WDM) is an alternate approach to increasing the data throughput of optical fiber systems. This approach, however, increases the aggregate bit rate simply by increasing the overall bandwidth utilized. It still suffers from bandwidth inefficiency. For example, a typical implementation of WDM might optically combine four OC-3 data streams, each at a different wavelength, to form an optical signal which has the same capacity as a single OC-12 data stream. The receiver would then optically separate the four OC-3 data streams, based on their wavelengths. In this approach, however, each OC-3 still has a bandwidth efficiency of approximately 1 bps/Hz, so the wavelength division multiplexed signal will also have a bandwidth efficiency of no more than 1 bps/Hz.
Thus, there is a need for systems and methods which transmit digital data over optical fibers at high aggregate data rates and with high bandwidth efficiencies, but without unnecessarily increasing the speed requirements on the corresponding electronics.
In accordance with the present invention, a system for transmitting digital data over an optical fiber includes a modulation stage, a frequency division multiplexer, and an optical modulator. The modulation stage receives a plurality of digital data channels and applies QAM modulation to produce a plurality of QAM-modulated signals. The frequency division multiplexer combines the QAM-modulated signals by frequency division multiplexing them into an RF signal. The RF signal is input to the optical modulator, which generates an optical signal modulated by the RF signal, for transmission over an optical fiber.
In a preferred embodiment, the modulation stage individually scrambles, forward error encodes and then QAM modulates, using 64 QAM modulation, each of 64 incoming OC-3 digital data channels to produce 64 QAM-modulated signals. The frequency division multiplexer combines the 64 resulting QAM-modulated signals in two steps, first frequency division multiplexing the QAM-modulated signals eight signals at a time to produce a total of eight signals at an intermediate frequency, and then frequency division multiplexing the eight intermediate signals to produce the RF signal. The optical modulator includes an optical source and an external modulator. The RF signal is applied to the external modulator to modulate the optical carrier produced by the optical source. The resulting optical signal is suitable for transmission across an optical fiber.
In accordance with another aspect of the invention a system for receiving digital data over an optical fiber includes a detector, a frequency division multiplexer, and a demodulation stage. The detector detects the optical signal produced by the transmitter system described previously, producing an RF signal. The frequency division demultiplexer separates the RF signal into its constituent QAM-modulated signals by frequency division demultiplexing. The demodulation stage converts the QAM-modulated signals into the original digital data channels.
The present invention is particularly advantageous because the combination of QAM modulation and frequency division multiplexing allows the transmission of digital data over optical fibers at high aggregate data rates and with high bandwidth efficiencies while using lower speed electronics. For example, the preferred embodiment described above has an aggregate data rate of approximately 10 Gbps and a bandwidth efficiency of approximately 4 bps/Hz, but the associated electronics need only support the 155 Mbps OC-3 data rate rather than the 10 Gbps aggregate rate.